Xylan is hemicellulose, which is the major component in plant cell wall, and also the second most abundant polysaccharides on earth. Therefore, the hydrolytic enzymes that degrade xylan are highly attractive and widely applied in many industries for a long time. Xylan is a long chain polysaccharide, which is composed of many pentose xylose units linked by β-1,4-glycosidic bond as a backbone of xylan. In nature, xylan is complex and highly branched heteropolysaccharide which can be decorated by methyl group or acetyl group, even branched by other sugar molecules to form various structures of xylan. Because of this complicated architecture of xylan, the complete degradation of xylan requires different xylanolytic enzymes to work together for decomposing xylan into simple sugars that can be used by organisms.
In general, xylanolytic enzymes can be divided into several groups including endo-β-D-xylanase, β-1,4-xylosidase, arabinase, acetylxylan esterase and α-glucuronidase. Among these enzymes, β-1,4-xylosidase (EC 3.2.1.37) is a crucial enzyme for complete degradation of xylan. It is an exoglucosidase that can hydrolyze the non-reducing ends of xylooligosaccharides into simple sugar xylose.
Since the xylosidase works together with the endo-xylanase and other xylanolytic enzymes, these enzymes can be cooperatively used in many different industries, such as bleaching process in paper industry, dough quality and juice clearance in food industry, animal nutrition in feed industry, even in biofuel production. According to different industrial needs, xylosidase is required to be suitable for different appropriate working conditions. In addition to the protein properties of enzyme, its catalytic efficiency is also the key point for improving industrial enzyme. Higher enzymatic activity represents the cost reduction in the industrial process and further enhances the commercial profit.
Currently, many researches try to obtain better enzymes by either screening in nature or modifying present enzymes. In the present invention, xylosidase is modified by rational design to increase its enzymatic activity, so as to further increase its application potential and economic value in industry.